For years, PWM inverters have been employed to convert DC into AC. In many applications, it is important to carefully regulate the quality of the AC emerging from the inverter. Accordingly, it was customary in the past to define a point of regulation ("POR") downstream of the inverter at which the voltage and current of the AC signal emerging from the inverter would be sampled. The sampled voltage and current would be used to aid in the selection of appropriate PWM switching patterns to minimize distortion at the POR.
Based on the voltage and current sensed at the POR, an inverter controller would select or create in real time an appropriate PWM pattern to ensure the least distortion at the POR. Obviously, as real and reactive power factors of electrical loads coupled to the inverter change, the PWM patterns fed to the inverter would change.
A PWM pattern comprises a set of switching transients which, when applied to a DC signal via an inverter, produces alternating pulses which, when filtered, become a sinusoidal AC signal. The pulses in a PWM pattern are of varying width. Normally, the inverter can adequately reproduce the PWM pattern and thereby create an accurate sinusoidal AC signal from the DC signal. However, under some load conditions (particularly when loads are unbalanced or have particular real or reactive power components), the inverter controller selects a PWM pattern which has switching transients defining pulses which can not be faithfully reproduced by the inverter due to physical limitations of transistor switches therein. In other words, the transistor switches within the inverter have physical limitations which manifest themselves, among other ways, in a minimum allowable switching time. Thus, should the inverter switch be called upon to switch faster than it is able, it will be unable to do so and therefore will be unable to faithfully reproduce a part of the PWM pattern required to maintain the least distortion at the POR.
Accordingly, under some conditions, the POR can not be regulated due to physical limitations of the transistor switches in the inverter.
Prior inventions have addressed schemes for controlling current and voltage at a POR. Representative of such inventions is U.S. Pat. No. 4,480,299, which issued on Oct. 30, 1984 to Muto et al. The patent to Muto et al is directed to a pulse width modulation inverter controlled by the use of the fundamental wave voltage E.sub.1 of the inverter output as a feedback quantity, the fundamental waveform E.sub.1 being obtained as the product of the function g(K.sub.h) which is determined by the amplitude ratio K.sub.h of a carrier wave and a modulated wave, and a DC voltage E.sub.d input of the inverter.
Muto et al is directed to control of an inverter by feeding back the fundamental wave voltage of the inverter output. Muto et al fails to show any apparatus for directly dealing with physical switching limitations within the inverter itself. Therefore, Muto et al may be subject to the same physical limitations to which the inverter of the subject invention is prone. Muto et al is not directed to modulation of the DC signal leading to the inverter.
The subject invention is the first to provide a DC to AC voltage inverter circuit having a variable voltage DC input such that PWM patterns having short duration switching transients are avoided. Accordingly, minimum switching time physical limitations of the switches within the inverter are avoided.
The DC input is varied by feeding back the AC output of the inverter. Feedback of the AC output allows inverter frequency to be changed while maintaining synchronization between the DC input variation and the PWM patterns fed to the inverter.